Static atmosphere ion beam accelerator having a movable target



STATIC ATMOSPHERE ION BEAM ACCELERATOR HAVING A MOVABLE TARGET Filed Aug. 25, 1965 Ifillll INVENTOR. REUBEN A. BE RGAN ATTORNEY 3,460,290 STATIC ATMOSPHERE ION BEAM ACCELERATOR HAVING A MOVABLE TARGET Reuben A. Bergan, Houston, Tex., assignor to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Filed Aug. 25, 1965, Ser. No. 482,409 6 Claims. (Cl. 31363) ABSTRACT OF THE DISCLOSURE A tritium target in D-T reactor is moved across ion beam by threaded bolt.

This invention relates to artificial sources of radioactivity, and more particularly relates to methods and apparatus for extending the useful life of static atmosphere ion beam accelerators adapted to produce radioactivity.

It is well known that various types of radiation can be generated by suitable nuclear reactions, and artificial sources of radiation are now commercially available which generate desired types of radiation by means of selected nuclear reactions. For example, 3.3 mev. neutrons may be generated with apparatus which bombards deuterium nuclei with high energy deuterons, and 14.4 mev. neutrons may be produced bybombardment of tritium with high energy deuterons. Similarly, 17 mev. gamma rays may be produced by bombardment of lithium nuclei with high energy protons.

All such artificial sources of radiation are commonly referred to as ion beam accelerators, since they basically involve the acceleration of the bombarding ions (deuterons, tritons, protons, etc.) against a target to achieve the nuclear reaction sought to be obtained. Such apparatus essentially consists of a source of ions, a target, and an accelerating means for accelerating the ions into the target to produce the sought for radiation. In a typical D-T reactor, the source of ions includes a pair of electrodes immersed in an atmosphere of deuterium and a power supply for passing a flow of electrons between the electrodes. The target consists of a metal body which is impregnated with tritium, and the accelerating means is a voltage supply for establishing a very high potential between the electrodes and the target. Deuterium ions created by the electrons are thus attracted or accelerated into the tritium-impregnated target, and a 14.4 mev. neutron is generated from each interaction between a deuteron and a triton.

It will be apparent that the radiation output of an ion beam accelerator will depend primarily upon the rate at which ions are produced, and upon the rate at which they are accelerated into the target. Thus, the atmosphere must be sufiiciently rich in the vicinity of the electrodes that ions can be created at a satisfactory rate, but also sufiiciently sparse between the electrodes and the target so as to support an adequate accelerating voltage. Thus, most ion beam accelerators are provided with systems which continually admit gas into the region between the electrodes, and also simultaneously pump away unionized gas from between the electrodes and the target. This type 'of accelerator is commonly known as a dynamic atmosphere accelerator.

There are many uses for an ion beam accelerator, however, where it is either impractical or impossible to include a pumping system of the type hereinbefore described. For example, it is often desirable to use an ion beam accelerator as a radiation source in radioactivity welllogging systems. However, well logging tools used in logging oil and gas wells must often be used in boreholes which are only five to six inches in diameter and thousands of feet deep. Not only is space at a premium, therenited States Patent fore, but all power and control signals must be supplied to the subsurface equipment, from the surface of the earth, by way of one or more electrical conductors in a flexible logging cable.

Accordingly, an improved ion beam accelerator which has a single sealed atmosphere has been developed primarily for use in well logging systems. This type of accelerator is commonly known as a static atmosphere" accelerator, since no pumping equipment is used to provide an atmosphere in the accelerating region which is different in density from the atmosphere in the ionization region. In a static atmosphere accelerator, the atmosphere is common to both regions, but is maintained at a very low pressure (density) so as not to impede acceleration of the atmosphere ions into the target. However, means such as magnets are provided in the ionization region to curve and greatly lengthen the path of the electrons so as to greatly improve the likelihood that they will strike the atmosphere atoms before reaching the anode member of the ionization electrode pair.

The static atmosphere accelerator has been a great success in that it can be made to produce substantial quantities of neutrons without any necessity for the complicated and elaborate pumping equipment used with the dynamic atmosphere accelerator. However, it is much more difiicult to control the density and purity of the atmosphere in a static atmosphere accelerator, and the amount of neutrons generated depends very directly upon the atmosphere in any accelerator being held within very narrow limits of density and purity. Establishing a static atmosphere within these limits of density and purity is a complex and time-consuming task. Thus, after a static atmosphere accelerator has been sealed, it is almost never reopened during its useful life.

The average static atmosphere accelerator is now a reasonably sturdy and dependable device. However, the target proper has a predictable lifetime, and when the hydrogen isotope in the target has been exhausted or burned away, the target must be replenished or replaced. Replacement of the target in a dynamic atmosphere is a relatively simple operation, since the tube atmosphere can be quickly and easily reestablished by means of the pumping system. However, it is impossible to replace the target in any accelerator without opening up its housing or jacket, and once the jacket of a static atmosphere accelerator has been opened, the accelerator must be completely reprocessed to reestablish the atmosphere within the aforementioned limits of density and purity.

These disadvantages of the prior art are overcome with the present invention, and novel methods and apparatus are provided which permit efiective replacement of the target of a static atmosphere accelerator without the necessity of opening up the accelerator jacket, and thus without adversely affecting the density and purity of the internal atmosphere of the static atmosphere accelerator.

Accordingly, it is an object of the present invention to provide novel methods and apparatus for extending the useful life of a static atmosphere accelerator.

It is also an object of the present invention to provide a novel method and apparatus for extending the useful life of the target in a static atmosphere accelerator without the necessity of opening up the accelerator jacket.

It is a particular object of the present invention to provide a novel method for extending the useful life of a static atmosphere ion beam accelerator having a housing, an internal atmosphere sealed gas-tightly in said housing, an ionization means disposed centrally in said housing for ionizing said atmosphere, a target disposed in said atmosphere adjacent said housing, and means for accelerating ions of said atmosphere into said target along a path extending between said ionization means and said target which is substantially narrower than said target, said method comprising selectably moving said target a preselected distance perpendicular to said path of said accelerated ions so as to expose to said ions a previously unexposed area of said target.

It is also a particular object of the present invention to provide in a static atmosphere ion beam accelerator having a housing, an internal atmosphere sealed gas-tightly in said housing, an ionization means disposed centrally in said housing for ionizing said atmosphere, a target disposed in said atmosphere adjacent said housing, and means for accelerating ions of said atmosphere into said target along a path between said ionization means and said target which is substantially narrower than said target, the improvement in combination therewith comprising means for selectively moving said target a preselected distance parallel to said housing and perpendicular to said path of said accelerated ions.

These and other objects and features of the present invention will be apparent from the following detailed description, wherein reference is made to the accompanying drawing.

In the drawing, there may be seen a partly cross sectional representation of a typical static atmosphere accelerator including one form of the preferred embodiment of the present invention.

All particle accelerators may be broadly classified as either columnar or radial accelerators. A so-called columnar accelerator is one in which the target is shaped in the manner of a fiat plate, and in which a columnarshaped beam of ions is directed at the target. A radial accelerator is one in which the ionizing electrodes are centrally mounted inside the jacket or housing of the accelerator, the target is ring or hoop-shaped and is mounted so as to surround the ionizing electrodes, and the ion beam is radial or disc-like in shape. Although the present invention is primarily useful in radial accelerators, it will be apparent that it may be readily adapted to columnar accelerators as well.

In its preferred form, the present invention is adapted for use in a radial accelerator having a round and cylindrically-shaped jacket which houses the target, ionization assembly, and the internal atmosphere sought to be ionized. As hereinbefore stated, the ionization assembly, is centrally mounted so as to :be disposed inside the standard, hoop-shaped target which is normally fixed to the inner surface of the jacket. In the present invention, however, there is an inner sleeve which is slideably disposed inside the jacket, and the target is therefore mounted on the inside surface of this sleeveboth the sleeve and the target being located so as to surround the ionization assembly.

The target in a typical radial-beam accelerator is approximately inch wide, whereas the ion beam is quite well focused and usually bombards only about 20% of the total *width of such a target. Thus, it is the function of the sleeve to provide a means for shifting the position of the target axially of the accelerator housing and the ionization assembly, so as to cause the beam to bombard a fresh area of the target after it has depleted the isotope content of the previously bombarded region or area.

The internal atmosphere of static atmosphere accelerator is sealed into the jacket at a very low pressure which is usually about 5-10 microns. Thus, if the aforementioned sleeve is moved so as to shift the target, such movement must be actuated in a manner which will not break the gas-tight seal around the atmosphere. In the preferred embodiment of the present invention, a moveable screw is provided which extends into the accelerator and is linked to the sleeve, and an accordian-like metal tube is disposed about the screw so as to provide a gastight barrier to separate the interior and exterior of the accelerator. In elfect, then, the accelerator jacket is basically provided with a flexible wall section by which motion can be applied to the target from a point outside the accelerator.

Referring now to the drawing, there may be seen a partly functional, partly pictorial representation of a typical radial static atmosphere accelerator having an internal atmosphere 2 composed of deuterium, a tritiumimpregnated target 4, and having an ionization assembly 6 mounted centrally inside a cylindrically-shaped jacket 8. The ionization assembly 6, which is connected to a socket 10 mounted in a high voltage insulator 12, is composed of a hollow metal tube 20 and a wire 22. The wire 22, which will hereinafter be referred to as the anode 22, is electrically insulated from the tube 20 which will hereinafter be referred to as the cathode 20. At one section, the cathode 20 may be seen to be composed of a wire mesh 21 located opposite the target 4.

The static atmosphere accelerator depicted in the drawing is adapted to be energized in a conventional manner by a Van de Graaff generator (not depicted) which creates a voltage between the mesh 21 and the anode 22 by delivering a charge flow to the anode 22. When sufficient charge has passed onto the anode 22, an electron flow will be produced between the mesh 21 and the anode 22, and any deuterium atoms therebetween which are struck by an electron will be converted into a positively-charged deuterium ion. Unfortunately, the internal deuterium atmosphere 2 is extremely sparse as has hereinbefore been explained. Thus, a plurality of magnets (also not depicted) are usually inserted equidistantly about the anode 22, and between the anode 22 and the mesh 21, so as to cause these electrons to have an extremely long path of travel and to greatly increase the likelihood that they will strike one of the deuterium atoms before reaching the anode 22. Accordingly, the rate at which deuterium ions are produced is greatly improved, notwithstanding the sparsity of the deuterium atmosphere 2.

The tritiumdmpregnated target 4 is electrically coupled to the steel jacket 8, and is therefore at ground or reference potential. Accordingly, the deuterium ions produced in the region between the anode 22 and the mesh 21 are accelerated through the mesh 21, and into the target 4, at very high speeds. Those deuterium ions which strike a tritium nucleus at a sufiicient speed will cause a nuclear reaction which, in turn, produces a 14.4 mev. neutron.

The apparatus depicted in the drawing is suitable for producing other types of nuclear reactions. For example, if the target 4 is impregnated with deuterium instead of tritium, the resulting deuterium-deuterium reaction will produce a 3.3 mev. neutron. Alternatively, if the internal atmosphere 2 is composed of protium (hydrogen-1), and if the target is composed of lithium or some other light metal, the resulting nuclear reaction will produce a 17 mev. gamma ray.

In any case, the accelerated ions tend to flow directly to the target 4 in a radial manner from around the circularly-shaped mesh 21. The resultant disc-like shape of the ion beam is further enhanced by the focusing or flattening effect provided by the ring-shaped suppressor electrodes 23 which are mounted on each side of the ion beam, and between the target 4 and the mesh 21, to suppress secondary electron emission by the target 4.

Accordingly, the radially-shaped ion beam tends to be concentrated on a thin section of the target 4, and thus the useful life of a static atmosphere accelerator depends, in essence, upon the number of tritium (or other) nuclei there may be in only a relatively restricted section of the target 4, rather than on the total supply of tritium (or other) nuclei there may be in the overall target 4. To put it another way, the effective size of the target 4 in any static atmosphere accelerator is determined by the width of the ion beam which bombards it.

In the conventional accelerator, the target 4 is mounted directly on the jacket 8. However, in the apparatus shown in the accompanying drawing, the target 4 may be seen to be inserted in a recess 24 in a cylindrically-shaped metal sleeve 26 which is snugly but slideably disposed inside the jacket 8.

At one end of the sleeve 26 there are attached .a pair of arms 28 which protrude through apertures 30 in a support plate 32, and which are affixed to a disc-like header bar 34 having an end fitting 36 mounted in its center. A coil spring 38 is mounted in a compressed condition between the end fitting 36 and the support plate 32, and is preferably attached to the end fitting 36. A threaded bolt 40 is fixedly imbedded at one end, in the opposite side of the end fitting 36, so as to extend into and through the center of an end plug 42 which is fixedly, and gas-tightly, mounted in an aperture in the jacket 8 at the opposite end of the accelerator from the insulator 12 and socket 10. As may be seen, the bolt 40' is preferably in axial alignment with the ionization assembly 6.

A round, internally-threaded nut 44, having a flangelike base, is rotatably mounted in a recess in the exterior surface of the end plug 42, and in threaded engagement with the other end of the bolt 40. A retaining ring 46 may be inserted in the end plug 42 to keep the nut 44 from slipping free of the end plug 42. It will be apparent that there is an unavoidable gas leak between the bolt 40 and the end plug 42. Thus, in order to provide a gas-tight seal between the exterior of the accelerator and its interior atmosphere 2, an accordian sleeve 50 is mounted in a gas-tight manner between the end fitting 36 and the end plug 42 so as to surround and house the bolt 40. The accordian sleeve 50 is a tubular, metallic member having a thin, accordian-like wall so that it may be axially extended or foreshortened without causing it to lose its general shape or form, and without creating any structural weakness in it.

As may be seen, rotation of the nut 44 will cause the bolt 40 to move axially of the accelerator in a direction which depends upon the direction of rotation of the nut 44. When the nut 44 is rotated so as to push the bolt 40, and consequently the end fitting 36, to its maximum point of extension into the accelerator and toward the ionization assembly 6, this will accordingly position the sleeve 26 towards the insulator 12 so as to position the upper edge region of the target 4 in line with the ion beam current produced by the ionization assembly 6. The term upper is used herein to define the edge region of the target 4 which is nearest the end fittting 36, since the accelerator is usually mounted in .a well logging instrument with the socket end in a downward position.

As hereinbefore stated, the ion beam is only about as wide as the standard target 4. Thus, after the upper edge region has become exhausted by usage, the nut 44 may be rotated a preselected number of turns (which will depend on the thread pitch of the bolt 40), and the sleeve 26 and target 4 may be shifted axially upward a distance equal to about 20% of the width of the target 4. Irt theory, this will permit the accelerator to be kept in service up to five times its present lifetime with a target 4 of conventional width.

It will be obvious that even greater service periods can be obtained if a wider target 4 is provided. In fact, in some cases it may be disirable to dispense entirely with the sleeve 26 as a separate means for supporting a very wide target 4, and the arms 28 may be attached directly to the target 4.

As hereinbefore stated, the rotation of the nut 44 can be preselected -to position the target 4 as desired. This also provides a means for determining the existing position of the target 4, since the metal jacket 8 of the accelerator prevents the target 4 from being visually exposed.

It should be apparent from the foregoing that the principle of the present invention is the axially shifting of the target 4 without breaking or penetrating the gas seal between the exterior of the accelerator and its internal atmosphere 2. Thus, in some adaptations of the accelerator, externally located magnetic means may be used to shift the target 4 or the sleeve 26 holding it. Alternatively,

an internally mounted bi-metal actuator means may be used which is responsive only to a temperature well in excess of that normally encountered in a borehole. However, the apparatus depicted in the accompanying drawing is deemed far superior to such alternatives, since it is both simple and dependable, and since it may be incorporated without substantial redesign of those accelerators presently in use.

However, these and numerous other variations and modifications may obviously be made without departing from the concept of the invention. Accordingly, it should be clearly understood that the forms of the invention described above and shown in the accompanying drawing are illustrative only, and are not intended to limit the scope of the invention.

What is claimed is:

1. In a static atmosphere ion beam accelerator having:

a housing having a longitudinal axis,

an internal atmosphere sealed gas-tightly in said housan ionization means disposed centrally in said housing for ionizing said atmosphere, a target disposed in said atmosphere adjacent said housing, and

means for accelerating ions of said atmosphere into said target along a path between said ionization means and said target which is substantially narrower than said target,

the improvement in combination therewith comprising:

means for selectively moving said target a preselected distance parallel to said longitudinal axis of said housing and perpendicular to said path of said accelerated ions while providing a substantially constant target area of ion bombardment.

2. In a static atmosphere ion beam accelerator having:

a metal housing having an aperture,

an internl atmosphere sealed gas-tightly in said housing,

an ionization means disposed at a point on the longitudinal axis of said housing for ionizing said atmosphere, and

means for accelerating ions produced by said ionization means radically and perpendicularly outwardly from said longitudinal axis of said housing along a narrow path having a predetermined width, the improvement in combination therewith comprising: a metal target slideably disposed in said housing parallel to said axis adjacent said ionization means providing a substantially constant target area of ion bombardment, said area having a width greater than the width of said path, shaft' having one end extending through said aperture in said housing and having the other end linked to said target, movement means for selectively moving said shaft longitudinally in said aperture of said housing, and a collapsible sealing means disposed about said shaft in gas-tight engagement with said other end of said shaft and said housing. 3. In a static atmosphere ion beam accelerator having: a cylindrically-shaped metal housing, an internal atmosphere sealed gas-tightly in said housan ionization means disposed at a point on the longitudinal axis of said housing for ionizing said atmosphere, and

means for accelerating ions produced by said ionization means radially and perpendicularly outwardly from said longitudinal axis of said housing along a narrow path having a predetermined width,

the improvement in combination therewith comprising:

a cylindrically-shaped metal target slideably disposed in said housing parallel to said axis adjacent said ionization means providing a substantially constant target area of ion bombardment,

said area having a preselected width greater than the width of said path,

nut means rotatably mounted in said housing in alignment with said axis,

threaded bolt means disposed generally inside said housing in alignment with said axis and having one end in screw engagement with said nut means and having the other end linked to said target,

a tube disposed gas-tighly about said bolt means and having collapsible walls.

4. In -a static atmosphere ion beam accelerator having:

a cylindrically-shaped metal housing,

an internal atmosphere sealed gas-tightly in said housan ionization means disposed at a point on the longitudinal axis of said housing for ionizing said atmosphere,

a target composed of a circular metal band having a predetermined width relative to said axis and containing nuclei of a preselected type, and

means for accelerating ions produced by said ionization means radially and perpendicularly outwardly from said longitudinal axis of said housing along a path having a width which is a predetermined fraction of said width of said target,

the improvement in combination therewith comprising a cylindrically-shaped metal sleeve slideably disposed in said housing and supporting said target in a spaced-apart relationship to said ionization means providing a substantially constant target area of ion bombardment,

nut means rotatably mounted in said housing in alignment with said axis,

threaded bolt means disposed generally inside said housing in alignment with said axis and having one end in screw engagement with said nut means and having the other end fixedly connected to said sleeve,

a tube disposed gas-tightly about said bolt means and having collapsible walls.

5. In a static atmosphere ion beam accelerator having:

a cylindrically-sh-aped metal housing,

an internal atmosphere sealed gas-tightly in said housan ionization means disposed at a point on the longitudinal axis of said housing for ionizing said atmosphere,

a target composed of a circular metal band having a pre-determined width relative to said axis and containing nuclei of a preselected type, and

means for accelerating ions produced by said ionization means radially and perpendicularly outwardly from said longitudinal axis of said housing along a path having a width which is a predetermined fraction of said width of said target,

the improvement in combination therewith comprising:

a cylindrically-shaped metal sleeve slideably disposed in said housing along said axis and having said target fixedly attached thereto to face said ionization means providing a substantially constant target area of ion bombardment,

nut means rotatably mounted in said housing in alignment with said axis,

threaded bolt means disposed generally inside said housing in alignment with said axis and having one end in screw engagement with said nut means and having the other end fixedly connected to said sleeve,

a tube disposed gas-tightly about said bolt means and having accordian-like walls.

6. In a static atmosphere ion beam accelerator having:

a cylindrically-shaped metal housing,

an internal atmosphere sealed gas-tightly in said housan ionization means disposed at a point on the longitudinal axis of said housing for ionizing said atmosphere,

a target composed of a circular metal band having a predetermined width relative to said axis and containing nuclei of a preselected type, and

means for accelerating ions produced by said ionization means radially and perpendicularly outwardly from said longitudinal axis of said housing along a path having a width which is a predetermined fraction of said width of said target,

the improvement in combination therewith comprising a cylindrically-shaped metal sleeve slideably disposed in said housing along said axis and having said target fixedly attached thereto to face said ionization means providing a substantially constant target area of ion bombardment,

plug means mounted gas-tightly in one end section of said housing in general alignment with said axis,

nut means rotatably mounted in said plug means in alignment with said axis,

threaded bolt means disposed generally inside said housing in alignment with said axis and having one end in screw engagement with said nut means,

fitting means fixedly attached to the other end of said bolt means,

connecting means fixedly linking said sleeve and said fitting means, and

a thin-Walled, metal tube disposed gas-tightly about said bolt means between said fitting means and said plug means and having accordian-like walls.

References Cited UNITED STATES PATENTS 2,926,270 2/ 1960 Zunick 313149 2,993,996 7/1961 Meyerhof 250--84.5 3,123,739 3/1964 Bergan 313-61 3,205,389 9/1965 Grimm 313146 3,229,145 1/1966 Jensen 313-446 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

